Nucleation method for atomic layer deposition of cobalt on bare silicon during the formation of a semiconductor device

ABSTRACT

A method used to form a cobalt metal layer on a silicon surface using an atomic layer deposition (ALD) process comprises a treatment of the silicon surface prior to cobalt formation. Treatment includes serial exposure to one or more cycles comprising a titanium nitride precursor or a tantalum nitride precursor, followed by an optional exposure to ammonia. After this treatment, the silicon surface is exposed to a metal organic cobalt such as cyclopentadienylcobalt dicarbonyl to form a cobalt precursor on the silicon surface, which is then exposed to hydrogen or ammonia to reduce the precursor to an ALD cobalt metal layer. Once this initial metal layer is formed, additional cobalt ALD layers may be completed to form a cobalt metal layer of a desired thickness.

FIELD OF THE INVENTION

This invention relates to the field of semiconductor manufacture and,more particularly, to a method for forming a pure metal cobalt layer ona bare, clean silicon surface using atomic layer deposition.

BACKGROUND OF THE INVENTION

During the manufacture of semiconductor devices such as dynamic randomaccess memories, static random access memories, logic devices, andmicroprocessors, several structures are commonly formed. For example,conductive interconnects such as word lines, and conductive plugs suchas digit line contact plugs, are commonly used.

A common engineering goal during the design of semiconductor devices isto manufacture as many features in a given area as possible. An obviousmethod to aid in accomplishing this goal is to make feature sizessmaller. One problem resulting from smaller feature sizes is thatdecreasing the width of a conductive line or conductive plug decreasesthe cross-sectional area of the line or plug, which in turn increasesthe resistance of the feature. Features which were originallymanufactured from only conductively-doped polysilicon, which has arelatively high resistance, required the formation of a lower resistancematerial to decrease the overall resistance of the feature, for examplean overlying layer of silicide such as tungsten silicide.

As device feature sizes further decrease it becomes desirable to formthe entire feature from a highly conductive material such as a metal,for example cobalt. Deposition techniques for metals include variousmethods. During one sputter process, a target manufactured from thedeposition material may be bombarded with ions to erode the materialfrom the target and redeposit it onto a semiconductor wafer substrateassembly. In another sputter process, the target material may be reactedwith gas phase species to form compound films. During chemical vapordeposition (CVD), gasses are mixed within a chamber and chemicallycombine on the wafer substrate surface to form a film. Both sputteringand CVD result in the continued increase in thickness of the materiallayer being deposited during the process. As long as the processcontinues the thickness of the layer increases. Both CVD and sputteringcan be used to deposit oxides, nitrides, and metals.

Thickness uniformity of a layer formed by either CVD or sputteringdepends on a variety of factors. For CVD, the gasses to be combined mustbe dispersed uniformly above the surface receiving the layer to bedeposited, otherwise the layer may form to a greater thickness in theareas of higher gas concentrations. The uniformity of a sputtered layeris highly dependent on the topography of the surface receiving thedeposited layer. Since the deposition process is line-of-sight, it isdifficult to deposit films in the bottom of a deep feature having a highaspect, or depth to width, ratio.

Another method used to form a layer is atomic layer deposition (ALD).With ALD, a precursor is introduced within a chamber to bond with freebinding sites on the surface of the wafer substrate assembly to form alayer which is a single atom or molecule thick. Once all the bindinglocations are full, chemical deposition stops regardless of how muchvapor remains in the chamber. This precursor gas is purged from thechamber and a reaction gas is introduced which reacts with the adsorbedprecursor until all surface sites are reacted, saturating the surfaceand regenerating a surface which will again react with the precursor.ALD has an advantage over CVD and sputtering in that it forms a highlyconformal layer over severe topography. ALD is typically used to formdielectrics such as oxides and nitrides, for example metal oxides andmetal nitrides. The deposition of some pure metals with ALD has alsobeen proven to be possible. However, the formation of pure metal cobalton bare silicon, for example by exposing bare silicon to a cobaltprecursor, has not proven possible. Cobalt can be formed on a baresilicon wafer but only after the wafer has been exposed to air for atleast 15 minutes. During this time the wafer surface becomes fullyhydrated, or the first monolayer of oxide is formed. Thus ALD cobalt cantypically only be grown on this thin oxide/oxygen layer. Oxide/oxygenformation has deleterious effects, for example increasing the resistancebetween the silicon wafer and a cobalt/cobalt silicide contact.

A method which enables atomic layer deposition of a metal such as cobalton atomically clean bare silicon would be desirable.

SUMMARY OF THE INVENTION

An embodiment of the present invention comprises a method of forming alayer of cobalt metal on bare silicon using an atomic layer depositionprocess.

In accordance with one embodiment of the invention a semiconductor wafersubstrate assembly is cleaned by exposure to a deoxidizer such ashydrofluoric acid to remove any native oxide or contaminants and toprovide a surface region of clean, bare silicon. Next, the wafer isplaced into a deposition chamber, or remains in a chamber from priorprocessing, and the wafer, specifically the bare silicon, is subjectedto one or more cycles of exposure to a tantalum nitride precursor or toa titanium nitride precursor, a purge of the chamber, and then anoptional exposure to ammonia. This process treats the bare silicon andprepares it to accept bonding with a cobalt precursor layer.

After such treating of the bare silicon, the deposition chamber ispurged and a cobalt precursor, particularly a metal organic cobalt suchas cyclopentadienylcobalt dicarbonyl, is introduced into the depositionchamber. After exposing the bare silicon to the cobalt precursor, thesilicon is exposed to ammonia to reduce the cobalt precursor to producea cobalt metal layer which may be used as a nucleation (seed) layer.

Additional cobalt formation acts may then be performed using thenucleation layer as a base to form a thicker cobalt metal layer, forexample, using cobalt ALD. If the feature is not already self-alignedfrom being formed on exposed silicon during a self-aligned silicideprocess (i.e. a “salicide” process), the cobalt metal may be patternedand etched to form a desired feature, or can be annealed to react thecobalt metal with the bare silicon to form a cobalt silicide feature.

This method may be used to form a semiconductor device having at leastone, and more commonly several, atomic monolayers of cobalt metaldisposed on a silicon surface such as a bare, unoxidized siliconsurface. This formation is believed to result from a structurecomprising a thin tantalum or titanium layer on the bare siliconsurface, terminated with dimethylamine groups. A subsequent exposure toammonia is believed to result in an in-process semiconductor devicehaving an NH₂-terminated silicon surface. A resultant structurecomprises a silicon substrate, at least one polysilicon feature thereon,and a layer of self-aligned cobalt silicide (i.e. “cobalt salicide”)covering the at least one polysilicon feature. Another resultantstructure comprises a silicon substrate, at least one polysiliconfeature thereon, a layer of cobalt silicide covering the at least onepolysilicon feature and a layer of cobalt metal covering at least aportion of the silicon substrate.

Advantages will become apparent to those skilled in the art from thefollowing detailed description read in conjunction with the appendedclaims and the drawings attached hereto.

Brief Description of the Drawings

FIGS. 1-4 are cross sections depicting the formation of a cobalt contactplug using an embodiment of the invention;

FIGS. 5-8 are cross sections depicting the formation of a self-alignedcobalt silicide structure;

FIG. 9 is an isometric depiction of various components which may bemanufactured using devices formed with an embodiment of the presentinvention; and

FIG. 10 is a block diagram of an exemplary use of the invention to formpart of a memory device having a storage transistor array.

It should be emphasized that the drawings herein may not be to exactscale and are schematic representations. The drawings are not intendedto portray the specific parameters, materials, particular uses, or thestructural details of the invention, which can be determined by one ofskill in the art by examination of the information herein.

Detailed Description of the Illustrated Embodiments

The term “wafer” is to be understood as a semiconductor-based materialincluding silicon, silicon-on-insulator (SOI) or silicon-on-sapphire(SOS) technology, doped and undoped semiconductors, epitaxial layers ofsilicon supported by a base semiconductor foundation, and othersemiconductor structures. Furthermore, when reference is made to a“wafer” in the following description, previous process steps may havebeen utilized to form regions or junctions in or over the basesemiconductor structure or foundation. Additionally, when reference ismade to a “substrate assembly” in the following description, thesubstrate assembly may include a wafer with layers including dielectricsand conductors, and features such as transistors, formed thereover,depending on the particular stage of processing. Further, in thediscussion and claims herein, the term “on” used with respect to twolayers, one “on” the other, means at least some contact between thelayers, while “over” means the layers are in close proximity, butpossibly with one or more additional intervening layers such thatcontact is possible but not required. Neither “on” nor “over” impliesany directionality as used herein.

A first exemplary embodiment of an inventive method to form a cobaltlayer on a bare silicon surface using atomic layer deposition (ALD) isdepicted in FIGS. 1-4. FIG. 1 depicts a semiconductor wafer substrateassembly comprising a monocrystalline silicon wafer 10 having a dopedregion therein 12 to which a cobalt metal contact will be formed. FIG. 1further depicts a dielectric layer 14 having an opening therein whichexposes doped region 12. The opening may be formed using a patternedphotoresist layer to etch the dielectric. It is likely that thecompleted FIG. 1 structure will comprise other features necessary fordevice functionality which are not immediately germane to the presentinvention and which, for simplicity of explanation, are not depicted ordescribed.

The dielectric layer may include one or more layers ofborophosphosilicate glass (BPSG), an oxide formed using tetraethylorthosilicate (TEOS), a combination of both, or another oxide such asspun-on glass (SOG). The FIG. 1 structure may be formed easily by one ofordinary skill in the art using standard processing techniques.

After forming the FIG. 1 structure, the wafer, particularly the baresilicon which forms the surface of doped region 12, is exposed to adeoxidizer such as hydrofluoric acid (HF) diluted about 100:1 withwater. This etchant solution will remove any native oxide (not depicted)and other contaminants from the bare silicon which forms the surface ofdoped region 12. Native oxide is known to form to about 12 angstroms (Å)depth on bare silicon. The wet etch will also remove about the sameamount of material from oxide 14, but negative effects are negligible.An alternative to HF is an acid such as QEtch II (1% phosphoric acid,H₃PO₄, and 39% ammonium fluoride, NH₄F) which will remove native oxidefrom bare silicon with minimal damage or contamination to the remainderof the semiconductor wafer substrate assembly. It is also contemplatedthat a dry etch may be used to remove any native oxide from the siliconsurface.

After exposing the silicon to the etchant to remove native oxide, thewafer is placed into a deposition chamber, or remains in such a chamberfrom prior processing steps. To prevent furnace hydration or thereformation of native oxide on the bare silicon after exposing the waferto HF, the wafer must be kept in an inert ambient such as N₂ or the nextprocess act must be initiated within about 15 minutes after etching awaynative oxide from the bare silicon.

Within the chamber the wafer is exposed to a tantalum nitride precursoror to a titanium nitride precursor. Titanium nitride precursors may havethe form Ti(NR₁R₂)_(x)(NR₃R₄)_(4-x)and Ti(NR₁R₂)₄, where R₁ through R₄are organic groups. Suitable titanium nitride precursors includetetrakis(dimethylamino) titanium (chemical formula Ti(N(CH₃)₂)₄,hereinafter “TDMAT”) and tetrakis(diethylamino) titanium (chemicalformula Ti(N(C₂H₅)₂)₄, hereinafter “TDEAT”). Tantalum nitride precursorswill typically have the form Ta(NR₁R₂)_(x)(NR₃R₄)_(5-x) where R₁ throughR₄ are organic groups. Suitable tantalum nitride precursors include(tert-Butylimino)tris (diethylamino)tantalum(V) (chemical formulaC₁₆H₃₉N₄Ta, hereinafter “TBDET”) and pentakis (dimethylamino)tantalum(V) (chemical formula Ta(NMe₂)₅, hereinafter “PDMAT” where “Me”is a —CH₃ methyl group). These precursors are available fromSigma-Aldrich Corp. of St. Louis, MO. The selected precursor used isintroduced into the deposition chamber at a flow rate of between about0.05 sccm and about 500 sccm for a duration of between about 10milliseconds and about 5 minutes. During the flow of, for example,TBDET, the chamber is maintained at a temperature of between about 200°C. and about 300° C. and a pressure of between about 1 millitorr andabout 20 torr. Other precursors having the general forms listed abovemay require an expanded chamber temperature range of between about 150°C. and 450° C. Exposing the silicon to the tantalum nitride (or titaniumnitride) precursor is believed to form a thin tantalum (titanium) layeron the surface of the silicon, terminated with dimethylamine groups.

After exposure of the wafer to TBDET the chamber is purged, for examplewith N₂, then ammonia (NH₃) is pumped into the chamber at a flow rate ofbetween about 5 sccm and about 5,000 sccm for a duration of betweenabout 1 second and about 30 seconds. The chamber temperature ismaintained to between about 200° C. and about 300° C. and a pressure ofbetween about 0.1 millitorr and about 20 torr. After exposing the waferto NH₃, the chamber is again purged, for example with N₂.

While the effect of the ammonia at this point in the inventive processhas not been determined, it is believed to remove the organic componentsof the precursor ligands to yield an NH₂-terminated surface. Exposure ofthe wafer to ammonia may reduce or eliminate carbon contamination,however, it may not be strictly necessary if a process results in theformation of a structure where carbon contamination at the interfacebetween the wafer surface and the subsequently-formed cobalt layer isnot a concern. However, it is contemplated that NH₃ exposure will beused in most applications of the inventive process.

The sequence of exposing the wafer to the tantalum or titanium nitrideprecursor, purging the chamber, exposing the wafer to NH₃, and againpurging the chamber is referred to herein as a Precursor/NH₃ cycle. Tomaximize bonding of the cobalt layer to the bare silicon, more than onePrecursor/NH₃ cycle may be performed, although one cycle is generallysufficient unless testing reveals enhanced benefits to the use ofmultiple cycles for a specific application.

After completing the Precursor/NH₃ cycle(s) the wafer is exposed to ametal organic cobalt such as cyclopentadienylcobalt dicarbonyl (chemicalformula C₅H₅Co(CO)₂, hereinafter “CpCo(CO)₂”). To expose the wafer,CpCo(CO)₂ is introduced into the deposition chamber at a flow rate ofbetween about 0.05 sccm and about 500 sccm for a duration of about 10milliseconds and about 5 minutes. The CpCo(CO)₂ may be introducedthrough an inert carrier gas at a flow rate of between about 0 sccm andabout 5,000 sccm. (As is known by one of ordinary skill in the art, at acarrier gas flow rate of 0 sccm some gas is injected into the chamberdue to the lower chamber pressure with low pressure processes.) Duringthe exposure the chamber is maintained at a temperature of between about200° C. and about 500° C., and the pressure is maintained at betweenabout 0.1 mT and about 20 torr. After CpCo(CO)₂ exposure the chamber ispurged using an inert gas at a flow rate of between about 0 sccm andabout 10,000 sccm for a duration of between about 0.01 seconds to about50 minutes. Exposing the bare, clean treated silicon surface to themetal organic precursor results in the precursor bonding with thesilicon surface.

After exposure to the metal organic precursor, a reducer such ashydrogen (H₂) or NH₃ is introduced into the chamber at a flow rate ofbetween about 5 sccm and 10,000 sccm for between about 0.01 seconds toabout 50 minutes. The H₂ or NH₃ reduces the cobalt precursor layerformed on the silicon surface to pure cobalt metal. While the mechanismhas not been studied, exposing the adsorbed cobalt precursor to NH₃ isbelieved to remove the organic ligand to yield metallic cobalt.

As a result of the treatment of the bare silicon with one or morePrecursor/NH₃ cycles to prime the exposed silicon surface with TaN orTiN, the surface termination of the bare silicon is altered and so thatit accepts bonding with the cobalt precursor. Without this initialtreatment, exposure of bare silicon to CpCo(CO)₂ does not result thebonding between the silicon surface and the cobalt precursor. Afterexposure of the treated surface to CpCo(CO)₂, a layer of adsorbed cobaltprecursor remains on the silicon surface. After exposure of the adsorbedcobalt precursor to H₂ or NH₃, the structure of FIG. 2 remains,including an ALD cobalt layer 20 on the bare silicon and the dielectriclayer 14. As this is an atomic layer, it forms a uniform conformalcobalt layer a single atom thick.

The cobalt metal will likely form as an incomplete layer covering about⅓ of the exposed surface after the initial layer of cobalt metal isformed. ALD cobalt metal formation may be continued by alternatingexposure of the cobalt metal layer to CpCo(CO)₂, then exposing thecobalt precursor which is adsorbed to the cobalt metal to H₂ or NH₃ toreduce the cobalt precursor to metal. The chamber temperature andpressure is maintained at the above-stated values during CpCo(CO)₂exposure and during exposure to the H₂ or NH₃ reducer. This cycling ofCpCo(CO)₂/NH₃ is performed until a cobalt layer of desired thickness isformed, for example as depicted in FIG. 3 which comprises a cobalt layer30 which completely fills the opening in dielectric 14.

Once the structure of FIG. 3 is formed, a planarization such as chemicalmechanical planarization (CMP) or other abrasive planarization may beperformed to remove the cobalt from the horizontal surface of dielectric14 to result in the cobalt plug 40 of FIG. 4. Wafer processing may thencontinue according to means known in the art to form a completedsemiconductor device.

A second exemplary embodiment of the invention is depicted in FIGS. 5-8in formation of a self-aligned silicide (i.e. “salicide”) ALD conductiveenhancement layer over a polysilicon feature. FIG. 5 depicts asemiconductor wafer 10 having a dielectric layer 50, such as a gateoxide or a thicker deposited oxide layer, formed over the semiconductorwafer 10. One or more silicon features, such as polysiliconinterconnects, transistor gates, etc., may be formed on the dielectriclayer 50 according to means known in the art. It is likely that thecompleted FIG. 5 structure will comprise other features necessary fordevice functionality which are not immediately germane to the presentinvention and which, for simplicity of explanation, are not depicted ordescribed.

Next, any native oxide is removed from the polysilicon features 52, forexample by exposure to one of the previously mentioned HF or QEtch IIetching solutions. Then, the polysilicon features are exposed to one ormore Precursor/NH₃ cycles using the conditions described above for thefirst embodiment of FIGS. 1-4. This sequence treats the polysiliconfeatures 52 such that the features will accept bonding with a cobaltprecursor.

Upon completion of exposure of the wafer to one or more Precursor/NH₃cycles, the wafer surface, specifically the polysilicon features 52, isexposed to CpCo(CO)₂ using the conditions described above for the firstembodiment, then the structure is exposed to a reducer (for example H₂or NH₃). This process sequence results in the structure of FIG. 6,having an ALD cobalt metal layer 60 formed over the wafer surface. Inthis embodiment, several additional layers of ALD cobalt are formed onthe initial layer by cycling CpCo(CO)₂/NH₃ until the cobalt layer isbetween about 5 Å and about 1,000 Å thick. As each cycle forms a cobaltlayer one angstrom (about ⅓ of an atom) thick on the previous layer(i.e. three or more passes are required to form a continuous, complete,and uniform layer), the CpCo(CO)₂/NH₃ cycles may be completed betweenabout 5 times and about 1,000 times.

After the cobalt metal reaches a desired thickness the wafer is annealedeither ex situ or in situ to react the metallic cobalt with thepolysilicon to form cobalt silicide 70 as depicted in FIG. 7. Oneexemplary anneal may include increasing the chamber temperature tobetween about 400° C. to about 600° C., for example about 500° C., forbetween about 2.5 minutes to 60 minutes, for example about 3 minutes.The chamber temperature may then be increased to between about 700° C.to about 900° C., for example about 800° C., for between about 20seconds to about 5 minutes, for example about 30 seconds. Pressure maybe maintained to between about 1 torr and about 760 torr, for example760 torr.

After forming the FIG. 7 structure, the unreacted cobalt metal layer 60is etched selective to the cobalt silicide 70 using hydrochloric acid toresult in the structure of FIG. 8, which comprises cobalt salicide layer80 formed over silicon features 52. Wafer processing then continuesaccording to techniques known in the art to form a completedsemiconductor device.

In another embodiment, the cobalt layer may remain on the wafer as anunreacted cobalt layer. One contemplated use for the unreacted cobaltmetal is as a portion of a digit line contact. Further, the inventivecobalt formation process may be performed subsequent to a gas phasecleaning process of the silicon wafer surface.

As depicted in FIG. 9, a semiconductor device 90 formed in accordancewith the invention may be attached along with other devices such as amicroprocessor 92 to a printed circuit board 94, for example to acomputer motherboard or as a part of a memory module used in a personalcomputer, a minicomputer, or a mainframe 96. FIG. 9 may also representuse of device 90 in other electronic devices comprising a housing 96,for example devices comprising a microprocessor 92, related totelecommunications, the automobile industry, semiconductor test andmanufacturing equipment, consumer electronics, or virtually any piece ofconsumer or industrial electronic equipment.

The process and structure described herein may be used to manufacture anumber of different structures which comprise a metal ormetal-containing feature. FIG. 10, for example, is a simplified blockdiagram of a memory device such as a dynamic random access memory havingdigit lines and other features which may be formed using an embodimentof the present invention. The general structure and operation of such adevice is known to one skilled in the art. FIG. 10 depicts a processor92 coupled to a memory device 90, and further depicts the followingbasic sections of a memory integrated circuit: control circuitry 104;row 106 and column 108 address buffers; row 110 and column 112 decoders;sense amplifiers 114; memory array 116; and data input/output 118.

While this invention has been described with reference to illustrativeembodiments, this description is not meant to be construed in a limitingsense. Various modifications of the illustrative embodiments, as well asadditional embodiments of the invention, will be apparent to personsskilled in the art upon reference to this description. It is thereforecontemplated that the appended claims will cover any such modificationsor embodiments as fall within the true scope of the invention.

1. A method for use in fabrication of a semiconductor device, the methodcomprising: providing a semiconductor wafer substrate assemblycomprising an exposed silicon surface; exposing the exposed siliconsurface to a one of a titanium nitride precursor and a tantalum nitrideprecursor to form a treated silicon surface; exposing the treatedsilicon surface to a metal organic cobalt to form a cobalt precursorthereon; and exposing the cobalt precursor to a reducer to form a cobaltmetal layer on the treated silicon surface.
 2. The method of claim 1further comprising exposing the treated silicon surface to ammonia priorto the exposure to the metal organic cobalt.
 3. The method of claim 2further comprising, prior to exposing the treated silicon surface to themetal organic cobalt and subsequent to exposing the treated siliconsurface to ammonia: again exposing the treated silicon surface to one ofthe titanium nitride precursor and the tantalum nitride precursor; andsubsequent to again exposing the treated silicon surface to theprecursor, again exposing the treated silicon surface to ammonia.
 4. Themethod of claim 1, further comprising selecting the metal organic cobaltto be cyclopentadienylcobalt dicarbonyl.
 5. The method of claim 1further comprising selecting the reducer to be a material selected fromthe group consisting of hydrogen and ammonia.
 6. A method for use infabrication of a semiconductor device, the method comprising: providinga semiconductor wafer substrate assembly comprising a silicon surfacehaving an oxide layer formed thereon; exposing the semiconductor wafersubstrate assembly to an etchant to remove the oxide layer to expose thesilicon surface; treating the exposed silicon surface with one of atitanium nitride precursor and a tantalum nitride precursor; exposingthe treated silicon surface to ammonia; exposing the treated siliconsurface to a cobalt precursor to form a cobalt precursor layer on thetreated silicon surface; and exposing the cobalt precursor layer to areducer to form an atomic layer of cobalt metal on the treated siliconsurface.
 7. The method of claim 6 wherein the semiconductor wafersubstrate assembly is exposed to a titanium nitride precursor selectedfrom the group consisting of a compound of the formTi(NR₁R₂)_(x)(NR₃R₄)_(4-x) and Ti(NR₁R₂)₄, where R₁, through R₄ areorganic groups.
 8. The method of claim 6 wherein the semiconductor wafersubstrate assembly is exposed to a titanium nitride precursor selectedfrom the group consisting of tetrakis(dimethylamino) titanium andtetrakis(diethylamino) titanium.
 9. The method of claim 6 wherein thesemiconductor wafer substrate assembly is exposed to a tantalum nitrideprecursor of the form Ta(NR₁R₂)_(x)(NR₃R₄)_(5-x) where R₁ and R₂ areorganic groups.
 10. The method of claim 6 the semiconductor wafersubstrate assembly is exposed to a tantalum nitride precursor selectedfrom the group consisting of (tert-Butylimino)tris(diethylamino)tantalum(V) and pentakis (dimethylamino) tantalum(V). 11.The method of claim 6 further comprising selecting the cobalt precursorto be cyclopentadienylcobalt dicarbonyl.
 12. The method of claim 6further comprising selecting the reducer to be a material selected fromthe group consisting of hydrogen and ammonia.
 13. A method for use infabrication of a semiconductor device, the method comprising: forming apatterned polysilicon layer over a semiconductor wafer substrateassembly; treating the patterned polysilicon layer by exposure to ametal nitride precursor; exposing the treated polysilicon layer toammonia; subsequent to exposing the treated polysilicon layer toammonia, retreating the polysilicon by exposure to the metal nitrideprecursor; exposing the retreated polysilicon layer to ammonia; exposingthe retreated polysilicon layer to a cobalt precursor to form a cobaltprecursor layer on the retreated polysilicon layer; exposing the cobaltprecursor layer to a reducer to reduce the cobalt precursor layer to acobalt metal layer; and annealing the semiconductor wafer to react thepolysilicon layer with the cobalt metal layer to form cobalt silicide.14. The method of claim 13 further comprising: forming the cobalt metallayer on a dielectric layer during the exposure of the retreatedpolysilicon layer to the reducer, wherein the cobalt metal layer on thedielectric layer remains unreacted subsequent to the anneal; andsimultaneously exposing the cobalt silicide and the cobalt metal to anetchant to remove the cobalt metal and to leave at least a portion ofthe cobalt silicide.
 15. The method of claim 14 further comprisingexposing the cobalt silicide and the cobalt metal to hydrofluoric acidduring the simultaneous exposure of the cobalt silicide and the cobaltmetal to the etch.
 16. The method of claim 13 wherein the cobaltprecursor layer is a first cobalt precursor layer, the cobalt metallayer is a first cobalt metal layer, and the method further comprises,prior to annealing the semiconductor wafer: exposing the first cobaltmetal layer to the cobalt precursor to form a second cobalt precursorlayer; exposing the second cobalt precursor layer to the reducer to forma second cobalt metal layer; repeatedly exposing exposed surfaces to thecobalt precursor and then to the reducer a sufficient number of times toform a conductive feature which is between 5 and 1,000 cobalt layersthick.
 17. The method of claim 13 further comprising selecting the metalnitride precursor to be a titanium nitride precursor selected from thegroup consisting of tetrakis(dimethylamino) titanium andtetrakis(diethylamino) titanium.
 18. The method claim 13 furthercomprising selecting the metal nitride precursor to be a tantalumnitride precursor selected from the group consisting of(tert-Butylimino)tris (diethylamino)tantalum(V) and pentakis(dimethylamino) tantalum(V).
 19. A method for use in forming aconductive feature during the fabrication of a semiconductor device,comprising: placing a semiconductor wafer substrate assembly into achamber, wherein the semiconductor wafer substrate assembly comprises asemiconductor wafer having an area to which contact with the conductivefeature is to be made; removing any oxide from the area by exposing thearea to a deoxidizer; treating the area by: exposing the area to one ofa titanium nitride precursor and a tantalum nitride precursor; exposingthe area to ammonia; introducing a metal organic cobalt into the chamberto form a cobalt precursor layer on the area; introducing a reducer intothe chamber to expose the cobalt precursor layer to the reducer and toreduce the cobalt precursor layer to cobalt metal; and repeatedlyintroducing the metal organic cobalt into the chamber then introducingthe reducer into the chamber a sufficient number of times to form theconductive feature from cobalt metal having about a selected thickness.20. The method of claim 19 further comprising selecting the metalorganic cobalt to be cyclopentadienylcobalt dicarbonyl.
 21. The methodof claim 19 further comprising performing the treatment of the areawithin fifteen minutes of exposing the area to the deoxidizer.
 22. Themethod of claim 17 further comprising selecting the reducer to be amaterial selected from the group consisting of ammonia and hydrogen. 23.(canceled)